It sounds like you may have parts created in different units.
Changing the units for an existing part is done through File > Prepare > Model Properties and in the dialog, under Materials > Units click on change. Once you select the units you want and click Set, you will be asked whether you want to Convert or Interpret. Convert re-scales the model so 1mm becomes 1". Interpret doesn't change the physical size so 25.4mm becomes 1 ".
If you want to change the default units (and drawing standards) there are bat files that will reconfigure Creo globaly for units and drawing standards. They are located here:
C:\Program Files\PTC\Creo 1.0\Common Files\M020\creo_standards
Does this answer your question?
Try creating a datum plane before hand (offset say 100mm), to help give some sense of scale to your part. If you have no features other than your 3 default datum planes then pro-e has no point of reference in terms of scale. Another thing you could try, is to sketch only 1 entity of your sketch to start with (say a line) & modify the length, height whatever, from this point on, you will be sketching something like the scale your require.
I'm in WF5 right now, but this functionality has to be similar in Creo.
1) Turn on the display of dimensions in sketcher. This will show all dimensions of the geometry you create. If this button is not visible once you go into sketcher, go to the TOOLS > CUSTOMIZE SCREEN > COMMANDS and go into sketcher categories and then find that button and drag it to your sketcher toolbar.
2) You can also turn on the grid in sketcher, which will give you a scale to start off all your sketches. The grid I show is made up of 30mm X 30mm squares. There are default values for this or you can go into the config options to set your own values.
Hope this helps.